1. Field of the Invention
The present invention relates generally to a radio communication system employing as a line multiplexing system a CDMA (Code Division Multiple Access) system which allows more ease in variable rate transmission than other speech coding systems for transmission and storage of speech information and a radio/wire communication system utilizing an ATM (Asynchronous Transfer Mode) switching system. More particularly, the invention pertains to a variable rate speech coding method and decoding method for storage of speech information, for instance, which are based on a CELP (Code Excited Linear Prediction) speech coding method and control whether or not to send sound source information parameters, thereby making the coding rate variable.
2. Description of the Prior Art
As one of conventional variable rate speech coding methods based on the CELP speech coding method, there is disclosed in Japanese Pat. Laid-Open Gazette No. 36495/95 a method that decides whether or not to transmit a sound source signal for each frame, thereby making its coding rate variable. FIG. 9 shows the coding procedure of each frame according to the conventional variable rate speech coding method. This coding procedure is carried out for each frame of a speech signal. That is, upon completion of the coding of the previous frame, the speech signal of the next frame is input and its coding starts with step SP1. In a linear prediction (hereinafter referred to simply as LP) analysis step SP2, an LP analysis of the speech signal is made to extract the speech signal of the current frame as an LP parameter representing spectrum information. Incidentally, the LP parameter is coded separately to be sent.
In the next sound source code book search step SP3, an adaptive sound source vector and a noise source vector are chosen so that a synthesized speech signal is obtained with a minimum distortion relative to the input speech signal. This is implemented by the use of an A-b-S (Analysis by Synthesis) method which, based on stored previous drive sound source vectors, selects an optimum combination of outputs from an adaptive sound source code book and a noise source code book that will minimize the distortion of the synthesized speech signal relative to the input speech signal of the current frame that is a reference speech signal. The input speech signal is obtained by adding together the adaptive and noise vectors and input into a synthesis filter which is constructed using a quantized version of the LP parameter obtained in step SP2 and from which the above-mentioned synthesized speech signal is output.
The adaptive sound source code book is one that outputs an adaptive sound source vector repeating the sound source signal at intervals of its the pitch period. The noise source code book stores and selectively outputs plural noise source vectors generated, for example, from random noise in a sequential order. Either code book holds therein a normalized version of the gain of the sound source in time sequence. Although the gain is usually computed separately and added to the sound source vector prior to transmission in coded form, the following description will be given on the assumption that each sound source vector contains the sound source gain. With the use of the A-b-S method, the synthesized speech signal is produced at the same time as the optimum combination of the adaptive sound source vector and the noise source vector is obtained.
In the next step SP4, a signal generated using only the adaptive sound source vector selected in SP3 is input into the same synthesis filter as in step SP3 to obtain therefrom a synthesized signal.
In the next step SP5, the synthesized speech signal quality is compared with a threshold value to decide whether or not to send a noise sound source code book index. By this, the variable coding rate is implemented. Step SP5 is composed of an SN ratio computing step SP5a of computing the SN ratio of a virtual synthesized speech signal relative to the input speech, a threshold value comparison step SP5b of comparing the computed SN ratio with a preset threshold value, a transmission suspending step SP5c of suspending the transmission of only the noise source code book index when it is judged in step SP5b that speech quality above the threshold value could be obtained even if the noise source code book index is not used, and an ordinary transmission step SP5d of transmitting all code book indexes.
Upon completion of the code transmission of the current frame in step SP5, the coding procedure of the frame is finished in step SP6 and the coding process for the next frame is started again with step SP1. In this way, the coding procedure is repeated for all the frames of the speech signal.
Incidentally, in the variable rate speech coding apparatus of the aforementioned Japanese Pat. Laid-Open gazette, even for synthesized speech obtained using the noise source vector alone, the transmission of the adaptive sound source code book index is suspended according to the result of an evaluation with the threshold value similar to the described above. Since the input speech period over which the above processing is performed is limited substantially to a silent duration during which no periodic information is generated, however, the processing does not contribute to improving the speech quality during a voiced steady-state period of speech.
The adaptive sound source code book in the CELP speech coding system has a role representing a periodic structure of speech based on its pitch period, whereas the noise source code book uses a noise component to compensate for a component that cannot fully be represented by the adaptive code book, that is, the remainder of the sound source information except periodic components. With the use of a sound source signal that is generated by adding together such components, it is possible to enhance reproducibility of an encoded sound source signal, permitting the generation of high quality synthesized speech.
With the variable rate speech coding method described above in respect of FIG. 9, only the adaptive. code book index is transmitted but the noise source code book index is normally restrained from transmission during a period with practically no periodicity of speech as in the case of the voiced steady- state period of speech. As referred to above, however, the noise source vector has the function of supplementing the periodic structure that cannot sufficiently be represented solely by the adaptive sound source vector. Without any noise source vector, the representation of the periodic structure would be insufficient, giving rise to a problem that the speech or tone quality in the voiced steady state period of speech is seriously deteriorated as compared with the speech quality when the synthesized speech is created by superimposing both the vectors one on the other.
With the method of the conventional apparatus with no structure for separately transmitting additional information, it is difficult to improve the speech quality in the input speech period during which the noise code book index is not transmitted but only the adaptive code book index is transmitted as mentioned above.
Moreover, the variable rate speech coding method of FIG. 9 computes the SN ratio of the synthesized speech based only on the adaptive sound source vector relative to the input speech signal in the concerned frame and compares the SN ratio with a preset threshold value to determine whether the noise code book index is to be transmitted or not. In the CELP speech coding system, however, coding is usually performed using a distortion minimizing standard for each frame, and consequently, the SN ratio of the synthesized speech signal greatly varies from frame to frame. Hence, with the criterion of judgement using the fixed threshold value, there are both cases where a code book index is transmitted and where it is not transmitted in accordance with the frame, depending on the SN ratio of the synthesized speech signal, for example, even during the steady-state period of speech--this results in the synthesized speech becoming unstable.